A conventional listening device such as a hearing aid includes, among other things, a microphone and a receiver. The microphone receives sound waves and converts the sound waves to an audio signal. The audio signal is then processed (e.g., amplified) and provided to the receiver. The receiver converts the processed audio signal into an acoustic signal and subsequently broadcasts the acoustic signal to the eardrum.
A receiver for a conventional listening device is shown in FIG. 1. As can be seen, the receiver 100 includes a housing 102 that protects sensitive audio signal processing circuitry inside the receiver 100. The housing 102 may be of a size and shape that allows the receiver 100 to be used in miniature listening devices, such as hearing aids. Terminals 104a and 104b located on the outside of the housing 102 allow the audio signal processing circuitry of the receiver 100 to be connected to other components in the listening device. Here, the terminal labeled 104a is the negative terminal which is connected to the system ground, and the terminal labeled 104b is the positive terminal.
A recent development in the field of listening devices in general and hearing aids in particular is the use of wireless communication. For example, it is now possible to program a listening device, such as a hearing aid, using radio frequency (RF) signals. The protocols for implementing such wireless communication are known to persons having ordinary skill in the art and will not be described here. In addition, two or more listening devices may now communicate directly with each other (e.g., for synchronization purposes) using a radio frequency link.
Listening devices such as hearing aids typically have very small batteries due to the reduced dimensions of the listening devices. Consequently, there is not a lot of power available for transmitting a radio frequency signal. The low power can result in a poor signal-to-noise ratio, which may render the listening devices extremely susceptible to interference. In some cases, even a moderate level of interference can disrupt the wireless communication, causing the programming or the synchronizing of the listening devices to fail.
One source of interference may be the receiver itself. For example, the audio signal processing circuitry in many modern receivers use a type of switching amplifier called a class D amplifier. These switching amplifiers are commonly used because they consume less power and are easier to implement than other types of amplifiers. Unfortunately, class D amplifiers are known to emit an electromagnetic signal having fundamental and harmonic frequencies that can interfere with the radio frequency signals received by the listening devices. And the housing or casing that encloses the audio signal processing circuitry is virtually transparent to the interference due to the material that it is made of. The problem is exacerbated by the close proximity of the receiver (and hence the class D amplifier) to the antenna of the listening device.
One possible solution is to provide a compensation coil around the receiver. A compensation circuit then supplies the compensation coil with a current that generates a counteracting field to the interference from the receiver. An example of this solution may be found in U.S. Published Application U.S. 20040028251 by Kasztelan et al. The Kasztelan et al. technique actively compensates for the interference by providing the compensation coil with an amplitude and phase-adjusted version of the original transmission signal. However, such a solution requires additional circuitry in the form of a compensation circuit, which makes the receiver more complex and costly to implement and occupies additional, already scarce space in the receiver.
A possible solution to the above problem is to implement some type of noise cancellation algorithm in the audio signal processing circuitry of the receiver. This solution, however, adds unwanted complexity to the operation of the listening device. And in any case, the electromagnetic signal emitted by the class D amplifier has a very unpredictable pattern, which makes it difficult to compensate for the interference using a noise canceling algorithm.
Accordingly, what is needed is a way to reduce or eliminate the interference emitted by the receiver in a listening device. Specifically, what is needed is a way to reduce or eliminate the interference in a manner that does not require any modifications to the audio signal processing circuitry of the listening device.